Dermatomyositis is a muscle disease that involves inflammation and a skin rash. Polymyositis is a similar inflammatory condition, that also involves muscle weakness, swelling, tenderness, and tissue damage but no skin rash. Both are part of a larger group of disease called inflammatory myopathy.
The cause of dermatomyositis is unknown. Experts think it may be due to a viral infection of the muscles or a problem with the body's immune system. It may also occur in people who have cancer in the abdomen, lung, or other parts of the body.
Anyone can develop this condition. It most often occurs in children age 5 to 15 and adults age 40 to 60. It affects women more often than men.
Symptoms may include:
The muscle weakness may come on suddenly or develop slowly over weeks or months. You may have trouble raising your arms over your head, getting up from a sitting position, and climbing stairs.
The rash may appear on your face, knuckles, neck, shoulders, upper chest, and back.
The health care provider will do a physical exam. Tests may include:
The main treatment is the use of corticosteroid medicines. The dose of medicine is slowly tapered off as muscle strength improves. This takes about 4 to 6 weeks. You may stay on a low dose of a corticosteroid medicine after that.
Medicines to suppress the immune system may be used to replace the corticosteroids. These drugs may include azathioprine, methotrexate or mycophenolate.
Treatments that may be tried when disease that remains active in spite of these medicines are:
When your muscles get stronger, your provider may tell you to slowly cut back on your doses. Many people with this condition must take a medicine called prednisone for the rest of their lives.
If a cancer is causing the condition, the muscle weakness and rash may get better when the tumor is removed.
Symptoms may go away completely in some people, such as children.
The condition may be fatal in adults due to:
The major causes of death with this condition are cancer and lung disease.
People with lung disease with the anti-MDA-5 antibody have a poor prognosis in spite of current treatment.
Complications may include:
Call your provider if you have muscle weakness or other symptoms of this condition.
Aggarwal R, Rider LG, Ruperto N, et al. 2016 American College of Rheumatology/European League Against Rheumatism Criteria for Minimal, Moderate, and Major Clinical Response in Adult Dermatomyositis and Polymyositis: An International Myositis Assessment and Clinical Studies Group/Paediatric Rheumatology International Trials Organisation Collaborative Initiative. Arthritis Rheumatol. 2017;69(5):898-910. PMID: 28382787 www.ncbi.nlm.nih.gov/pubmed/28382787.
Dalakas MC. Inflammatory muscle diseases. N Engl J Med. 2015;373(4):393-394. PMID: 26200989 www.ncbi.nlm.nih.gov/pubmed/26200989.
Nagaraju K, Gladue HS, Lundberg IE. Inflammatory diseases of muscle and other myopathies. In: Firestein GS, Budd RC, Gabriel SE, McInnes IB, O'Dell JR, eds. Kelley and Firestein's Textbook of Rheumatology. 10th ed. Philadelphia, PA: Elsevier; 2017:chap 85.
National Organization for Rare Disorders website. Dermatomyositis. rarediseases.org/rare-diseases/dermatomyositis/. Accessed April 1, 2019.